Instinctual
behavior in which social bonds are formed during early development. Typically
used with the example of a newly hatched bird or other animal identifying
and treating the first moving object it sees as its mother.

incomplete dominance

Occurs when two different alleles of the same gene are
both partially expressed in a heterozygote. The resulting phenotype
is intermediate between the homozygous phenotypes of the two alleles.
Color in a four o’clock flower is an example of incomplete dominance.

independent assortment

Mendel’s conclusion that during gamete formation, the
segregation of one pair of genes has no influence over the segregation
of another pair of hereditary units. Two different traits will thus
be inherited independently of each other. This law holds true, except
for linked genes. It is often called Mendel’s second law.

inheritance of acquired traits

A crucial, and incorrect, aspect of Lamarck’s theory
of evolution. Lamarck believed that the traits a parent acquired
during its lifetime would be passed on to its offspring.

instincts

Behavior
that is hardwired into the brain of an organism; behavior that does
not have to be learned.

insulin

Hormone
secreted by the pancreas that reduces blood sugar levels. A lack
of insulin can result in diabetes.

interphase

The
phase of the eukaryotic cell cycle in which the cell prepares for
division, primarily by replicating its DNA. After interphase, the
cell enters mitosis.

intertidal zone

The
most shallow zone in a marine habitat. Periodically dry or wet with
the changing tides. Algae, sponges, mollusks, starfish, and crabs
inhabit this zone. Also called the littoral zone.

ion

An atom
or molecule that has lost or gained an electron and consequently
has a positive or negative charge.

isotonic

A
situation in which the concentration of solutes in a solution is
equivalent to what it contains. There is no concentration gradient
in isotonic solutions and no net flow of solutes or water.

K

karyotype

A
photograph of the chromosomes from an individual cell, usually lined
up in homologous pairs, according to size. Missing, extra, or abnormal
chromosomes can then be easily identified, aiding in the diagnosis
of genetic disorders.

kidney

The
organ of blood filtration in humans. The functional unit of the
kidney is the nephron.

kingdom

The
broadest category of biological taxonomy.

Krebs cycle

A
metabolic pathway in aerobic organisms in which acetyl-CoA combines
with oxaloacetic acid to form citric acid. For this reason, the
Krebs cycle is also known as the citric acid cycle. The cycle produces
energy in the forms of ATP, NADH,
and FADH2.

L

Lamarckism

An
evolutionary theory (proved false by Darwin) stating that species
change over time by the use and disuse of structures and the inheritance
of acquired traits.

learned behavior

Behavior
that an organism picks up over the course of its life. Three types
of learned behavior are habituation, conditioning, and associative
learning.

Leeuwenhoek, Antonie van

Dutch clothing salesman (1632–1723) who made the first observations
of bacteria and protozoa using single-lens microscopes of his own
design.

ligament

Connective
tissue between bones.

linked genes

Genes
that are located close together on the same chromosome. Linked genes
will not undergo independent assortment during gamete formation,
constituting an exception to the law of independent assortment.
Crossing-over will disrupt the linkage of two genes on the same
chromosome if they are far enough apart.

lymphatic system

A
means of returning blood fluid, lymph, that has escaped from capillaries back
into the bloodstream. Defended against foreign bodies by lymphocytes.